Micromechanical rate-of-turn sensors are used, e.g., in motor vehicles for the functionality of the electronic stability program, ESP, or for roll-tendency compensation. They use the Coriolis effect to measure, e.g., the rate-of-turn around the vertical axis or longitudinal axis of the motor vehicle.
The description below pertains only to the driving circle that occurs in rate-of-turn sensors of this type, without the measuring circle for determining the Coriolis acceleration.
Micromechanical rate-of-turn sensors contain one or more mechanical oscillators, each of which includes an elastically suspended oscillator element. The mechanical oscillators can be stimulated to perform a periodic oscillating motion using driving forces that change periodically with time and are applied electrostatically.
Rate-of-turn sensors are known, e.g., from DE 102 37 410 A1 and DE 102 37 411 A1.
Rate-of-turn sensors are operated at a mechanical driving frequency of the oscillator element. To this end, a suitable stimulation frequency of a drive signal F(t) that generates the driving forces must be selected. At the driving frequency, a phase shift does not occur between the speed of the oscillator element and the stimulation frequency of the drive signal F(t), and the phase shift between the deflection of the oscillator element and the stimulation frequency of the drive signal F(t) is −π/2. Ideally, the shape of the graph of the drive signal F(t) that generates the driving forces is sinusoidal. The term “sinusoidal” also refers to a graph with a cosinusoidal shape, i.e., a sinusoidal curve shifted by π/2. A square-wave drive signal F(t) that is easy to generate is usually used in practical application, however.
The instantaneous deflection or the instantaneous speed of the oscillator element is measured using capacitive sensors located on the oscillator element, and a signal x(t) proportional to the instantaneous deflection, or a signal v(t) proportional to the instantaneous speed is output. When the signal v(t) proportional to the speed is amplified in a suitable manner, the resultant signal can be used once more—either directly or after squaring—as a drive signal F(t) for generating the driving force. If deflection is measured instead of speed, the phase shift of −π/2 must first be compensated, e.g., using an additional phase modifier, before it is coupled with the drive signal F(t).
In both cases, the mechanical oscillator begins to oscillate with a driving frequency when sufficiently amplified. To ensure that the oscillation amplitude does not become too great, and to prevent damage to the oscillator element, the oscillation amplitude must be measured and regulated to a specified value by changing the amplitude AF of the drive signal F(t).
Since a mechanical oscillator can have higher-frequency, spurious driving frequencies in addition to its main mechanical driving frequency, cases arise in which the arrangement described above does not oscillate at the main driving frequency, but rather at a spurious driving frequency. This is a serious problem, because the functionality of the rate-of-turn sensor becomes unavailable as a result.
This effect can occur in particular when the drive signal F(t) is square-wave. Since x(t) or v(t) are sinusoidal signals, they are typically forwarded to a comparator, which compares the signals with zero and outputs a square-wave output signal F(t). With small input signals x(t) or v(t), the amplification of a comparator of this type reaches very high values, i.e., of a magnitude greater than 105. As a result, it is also possible to stimulate very weak spurious resonances in the oscillation motion of the oscillator element to become unwanted, stable oscillations.
In practical application, methods are used in some cases with which the signals x(t) or v(t) are not coupled directly with the drive signal F(t). Systems of this type function in a manner similar to a phase locked loop, PLL. An example of a system of this type is shown in FIG. 1, framed by a dashed line. In this case as well, the system can lock into a spurious resonance if it is not dimensioned properly and/or if it is not monitored.